include/devicetree: clocks: Add DT_CLOCKS_HAS_FOO & DT_NUM_CLOCKS

Add macros DT_CLOCKS_HAS_NAME and DT_CLOCKS_HAS_IDX.
These macros allow to check the presence of a specific clock name
or a specific cell at a given index in a clocks property.
Matching _INST_ macro variants are also provided.

Also add DT_NUM_CLOCKS and its _INST_ variant that counts
the number of clocks available for a given node.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>